Wii Fit

My wife and I live in the UK and we enjoy using the Wii Fit for fairly gentle exercise [we are both in our late 70s].
One thing puzzles me concerning the steps exercise.I manage to get 'OK' most times when I step on and off the balance board but I don't often get the 'Perfect'.What am I doing wrong? When I watch my wife I can't see what the difference is between getting 'OK' and'Perfect'